1. Field of the Invention
The present invention relates to an apparatus and method for controlling data transmission rate, taking into account collision in a Wireless Local Area Network (WLAN), in which a collision-caused rate decrease is reduced by detecting collision-caused data transmission failures, and if a predetermined number of or more data transmission failures occur, data is retransmitted by a Request-To-Send/Clear-To-Send (RTS/CTS) exchange.
2. Description of the Related Art
An Institute of Electrical and Electronics Engineers (IEEE) 802.11 WLAN adopts a link adaptation strategy to enhance throughput for a Station (STA). The link adaptation strategy selects the highest available transmission rate at a given time among the multiple transmission rates provided by IEEE 802.11 adaptively according to the current link condition.
While the IEEE 802.11 WLAN standards specify the multiple available transmission rates, how to use them, i.e. the link adaptation strategy is yet to be specified. In this context, many link adaptation schemes have been proposed in these or many other forms. Automatic Rate Fallback (ARF) is the first link adaptation algorithm proposed for use in the IEEE 802.11 WLAN and most widely implemented for STAs and Access Points (APs) at present.
In ARF, in order to estimate the highest available data rate under the current radio environment, the transmission rate is decreased by one level if transmission failure occurs twice successively, and raised by one level if the number of consecutive successful transmissions reaches 10. For details of ARF, see WaveLAN-II: A High Performance Wireless LAN for the Unlicensed Band, by Ad Kamerman and Leo Monteban, Bell Labs Technical Journal, vol. 2, no. 3, pp. 118-133, August 1997.
A drawback with most of the so-far proposed link adaptation schemes including ARF is that performance decreases drastically in an environment suffering from high radio resource contention. When many STAs associate with an AP or under severe contention, the probability of collision with frames from another STA or from another AP increases. However, since the Medium Access Control (MAC) layer of the IEEE 802.11 WLAN assumes a transmission failure solely after a timeout period, it does not distinguish a collision-caused transmission failure from a channel error-caused transmission failure. Therefore, in a link adaptation scheme such as ARF, when a collision occurs, the collision is mistaken as channel errors and the transmission rate is one level decreased. This malfunction is more frequent as contention becomes more severe. Even under a radio environment allowing data transmission at higher data rates, the rates of STAs and APs attempting to send at lower rates increase.
When channel errors are generated due to conditions such as path loss, fading or frames colliding with each other, a sending STA fails to receive an Acknowledgement (Ack) frame for a transmitted data frame. Time consumption involved in failed transmission increases with a larger data frame size and a lower transmission rate. Especially in the presence of a hidden STA or when more STAs contend for a radio channel, the problem becomes worse, adversely affecting the entire WLAN.
The RTS/CTS exchange implemented and used basically in the IEEE 802.11 WLAN family of technologies is a strategy for controlling collision using short control frames without payload, RTS and CTS frames.
Before data frame transmission, an RTS frame is sent to a receiving STA. After a Short InterFrame Space (SIFS) defined by the IEEE 802.11 WLAN standards, the receiving STA replies with a CTS frame. Also after the SIFS, a sending STA sends an actual data frame. The RTS and CTS frames contain information indicating the transmission times of the actual data frame and an Ack frame for the data frame, all other STAs hearing the RTS/CTS packet defer channel access during the RTS-CTS-data-Ack frame transmission. As a consequence, the sending STA reserves the channel for the duration of data frame transmission by the RTS/CTS exchange. Hence, the hidden STA problem is solved. Even when the RTS frame is lost due to collision, the loss is relatively small because the RTS frame is relatively short. Therefore, the RTS/CTS exchange mechanism is highly effective in an environment where many STAs contend for the channel.
However, the RTS/CTS exchange mechanism adds to overhead with transmission of RTS/CTS frames. Since the RTS/CTS exchange overhead increases with less STAs participating in contention, this scheme is not used much in a real WLAN environment.
Accordingly, there exists a need for an apparatus and method for sending data at an optimum transmission rate both in a high-contention environment and a low-contention environment of a WLAN.